An optical isolator, sometimes referred to as an optical coupler, may be used in an electrical circuit to allow signal propagation in a forward direction while maintaining voltage and current isolation between the input and the output of the optical isolator. Such an isolator includes a light emitting diode (LED) at the input, a photodetector at the output and an optically transmissive, high breakdown-voltage isolation gap separating the LED and the photodetector. For optimum performance, reliability and reproducibility, the geometry and materials must be maintained accurately and reproducibly during manufacture.
It is also important for obtaining high signal strength to provide a high degree of light transmission between the input and output. It is desirable to have an efficient LED with high light output as a function of current. It is also desirable to have a sensitive photodetector with a large photodetection area to capture as much light as possible. Of considerable significance, however, is minimizing loss of light emitted by the LED before the light reaches the photodetector.
Another important consideration in a optical isolator is the degree of isolation of the input voltage from the output. Until recently isolation voltages of up to about 2,500 volts have been acceptable. In some applications, performance requirements seek electrical isolation of 5,000 volts or more. The physical dimensions of the parts are extremely small so excellent electrical breakdown resistance must be provided to maintain such voltage isolation.
It is convenient to provide an optical isolator as a separate component which can be employed in a variety of circuit applications. Thus, such components are conveniently provided in dual in-line packages or DIPs where the device is encapsulated in a molding compound with a row of electrical leads along each edge so that the DIP can be attached to a printed circuit board or the like. U.S. Pat. No. 4,694,183 by Merrick, et al., describes such a DIP and its manufacturing technique.
In such an embodiment the optical isolator is fabricated on an almost flat metal lead frame. A portion of the lead frame is "folded" to place the LED opposite the photodetector. An optically transmissive silicone gel is placed between the LED and photodetector to keep a clear optical path therebetween. This assembly is then encapsulated in conventional opaque transfer molding material and the metal leads are trimmed and bent to complete the DIP.
Surface emitting LEDs have been used in such an embodiment. Light is emitted from a surface of an LED which is aligned with the photoreceptive surface of the photodetector. However, it is desirable to employ an isotropic emitting LED which emits light in all directions so that a larger light flux may be obtained from a smaller device. As much as 40% of the light from an isotropic LED is emitted from the sides and in an arrangement as provided in U.S. Pat. No. 4,694,183, much of that light is lost from the silicone gel into the surrounding molding compound.
The amount of light transmitted from an isotropic LED toward the photodiode may be increased about 30% (as measured by the photodiode current) by forming a shallow reflective cup in the metal lead on which the LED is placed. Thus, light emitted laterally from the LED is reflected in the direction of the photodetector. It is desirable to enhance the amount of light transmitted from the LED to the photodetector even more. In practice of this invention, the total light flux has been increased to 300% of the light flux without this invention.
It appears that electrical breakdown and arcing between the input and output occurs along interfaces between materials, particularly when the interface is not well bonded. Thus, for example, there may be poor bonding between the transfer molding material used for encapsulating the DIP and the silicone used to provide a clear light path between the LED and photodetector. Silicone resins have the property of wetting most surfaces quite well before curing, but after curing they become resistant to wetting by other materials. Since the transfer molding material does not wet the silicone well, poor bond strength is obtained, leaving an electrically weak path along which electrical breakdown and arcing may occur. It is therefore desirable to provide a means for assuring good bonding and high resistance to electrical breakdown. Isolation voltages in excess of 5000 volts are obtained in practice of this invention.